It is common knowledge that the most complicated technical problem in electric loudspeaker systems is reproduction of sound at the lowest frequencies. In the vast majority of cases electrodynamic radiators are used as radiators at low frequencies. In such radiators a dramatic drop of sound pressure frequency response (FR) takes place at frequencies lower than the fundamental resonance frequency which is accounted for by their physical principles of operation. This is particularly topical in small-size loudspeaker systems where the diameter of the radiator diffuser and the dimensions of the loudspeaker system are confined within strict limits. These limits do not allow creating a loudspeaker system with the fundamental resonance frequency which would be sufficiently low.
One of possible solutions to this problem has been suggested in “Loudspeaker distortion reduction systems” patent, U.S. Pat. No. 4,052,560, John Bryant Santmann, Oct. 4, 1977. This patent suggests that a low-pass filter (LPF) should be introduced before the power amplifier. The LPF provides a rise of the amplifier FR and is able to compensate for the drop of the loudspeaker system sound pressure FR at low frequencies completely. The main disadvantage of this method is an excessive correction at the lowest frequencies. A specific feature of most low-frequency electro-acoustic transducers is their extremely low efficiency at frequencies that are more than 2.5-3 times lower than the cutoff frequency. Deep correction of the amplifier FR is not efficient at these frequencies; it results in a high level of non-linear and transient distortions and limits the actual maximum power of the loudspeaker system significantly. Therefore, this method of correction can be used for loudspeaker systems whose initial cutoff frequency does not exceed 60-70 Hz (it is assumed that the actual sound signal does not have any components with a considerable level at frequencies below 25-30 Hz). Most small-size loudspeaker systems do not meet these requirements. Another disadvantage is the necessity of introducing additional amplifying elements of the integrator which are to provide a rise of the amplifier FR 12 dB/octave and more to form FR which is close to the horizontal sound pressure FR. This sophisticates the amplifier and increases its cost.
Another well-known solution to the problem of low frequency reproduction is the use of acoustic transducer current positive feedback (PFB) in the power amplifier. This PFB produces negative output impedance of the amplifier and is able to provide a rise of the FR at low frequencies by means of introducing a low-pass filter into the PFB circuit. This idea can be implemented in numerous ways involving different frequency correction circuits and different quantity of amplifying elements. There is an arrangement similar to the claimed invention. It is described in “Acoustic apparatus with secondary quarterwave resonator” patent, U.S. Pat. No. 5,313,525, Michael A. Klasko, May 17, 1994, FIG. 18, p. 11. The main disadvantage of this circuit is the fact that the rise of the amplifier FR has low steepness and the correction depth is limited by the system stability conditions. This does not allow achieving any substantial extension of the FR horizontal area.
Another method of correction at low frequencies is provided in “Loudspeaker lower bass response using negative resistance and impedance loading” patent, U.S. Pat. No. 4,118,600, Karl Erik Stahl, Oct. 3, 1978. This solution is based on an amplifier with significant active negative output impedance and a considerable proportion of reactive output impedance which changes the speaker equivalent characteristics (e.g. the movable system equivalent mass) for the purpose of considerable extension of the range of reproducible low frequencies. A disadvantage of the system is that the rise of the reproduction level at frequencies below the loudspeaker system fundamental resonance frequency is accompanied by deterioration of reproduction at higher frequencies. Another disadvantage is a need for additional amplifying elements.